Conventionally, fluorescent lamps have been mainly used as a light source for illumination, and illumination apparatuses turned on by using a high-frequency inverter switching device have been widely spread. Recently, light emitting diodes (LEDs) are spotlighted as an electrical light source other than discharge lamps such as fluorescent lamps. In particular, since the LEDs have a relatively longer lifetime than fluorescent lamps, they are expected to become superior to the fluorescent lamp FHF32 mainly used for base lighting by future technological improvement.
As LED technology improves, there is developed a light source module with LEDs mounted thereon. In the light source module, it needs to determine both the number of the LEDs to use therein and whether to connect the LEDs in series or in parallel in order to achieve a constant light output from the light source module. That is, the number of the LEDs to use and the connection arrangement is determined in design of the light source module such that current and voltage values of the light source module are appropriately set.
Furthermore, a lighting apparatus for supplying current to the light source module is designed to generate an appropriate output to save power with improvement of LED technology. However, as described above, the current and voltage values of the light source module vary depending on electrical characteristics of each LED, the number of the LEDs in use and whether the LEDs are connected in series or in parallel. Despite the improvement of LED technology, the light source module needs to be designed to have a specific combination of the characteristics of each LED, the number of LEDs in use and the connection arrangement by which can generate constant current.
For example, when an LED with a voltage characteristic of 3.5 V is used, a lighting apparatus applies a voltage of 17.5 (=3.5×5) V to a light source module (hereinafter, referred to as an “LED module”) having 5 LEDs with this characteristic connected in series. If 4 LEDs with the same characteristic connected in series are connected to the illuminating device, an overvoltage is applied, resulting in over-current.
Japanese Patent Application Publication No. 2009-224046 (hereinafter, referred to as “Reference 1”) discloses a notification terminal for notifying the connection and disconnection of an LED module as a means to prevent a breakdown caused by such excessive current, thereby preventing excessive current based on a notification signal from the notification terminal. Furthermore, Reference 1 discloses a configuration capable of providing a constant current to the LED module.
Reference 1 considers the difference in the number of the LEDs in use but does not consider the improvement of LED technology as mentioned above. For example, an LED with a voltage characteristic of 3.5 V and a current characteristic of 0.3 A is considered. The voltage applied to an LED module including 10 LEDs with such characteristics connected in series is 35 (=3.5×10) V and the output current thereof is 0.3 A. If an LED with a voltage characteristic of 3 V and a current characteristic of 0.2 A becomes available through the improvement of the LED technology, the voltage applied to an LED module having 8 LEDs with such characteristics connected in series becomes 24 (=3×8) V.
Therefore, when it is compared to the LED module including 7 LEDs with a voltage characteristic of 3.5 V connected in series to which a voltage of 24.5 (=3.5×7) V is applied, the application voltage difference caused by the differences in voltage characteristics and the number of the LEDs in use is not substantially large. However, if 0.3 A flows through an LED module with an output current of 0.2 A, abnormal heat is generated due to the over current, resulting in a breakdown or lifetime reduction.
In Japanese Patent Application Publication No. 2009-283281 (hereinafter, referred to as “Reference 2”), there are 3 types of LED modules, each being different in the number of LEDs connected in series. When one of the 3 LED modules is connected to an illuminating device, the lighting apparatus applies small current to the LED module and determines the type of the LED module based on a voltage drop in the LED module. Then, a voltage applied to the LED module from the lighting apparatus is controlled on the basis of the determination result. Therefore, Reference 2 has also the same problem as Reference 1.
In Japanese Patent Application Publication No. 2009-21175 (hereinafter, referred to as “Reference 3”), an LED module is provided with a storage unit for storing information on a current characteristic of the LED module which varies on type of LED module. When a lighting apparatus is connected to the LED module, an information monitoring unit of the lighting apparatus reads the information on the current characteristic from the storage unit of the LED module. Then, the lighting apparatus controls a voltage to apply to the LED module according to the current characteristic information read by the information monitoring unit.
By utilizing the technology disclosed in Reference 3, a lighting apparatus responding to future technological improvement of LEDs can be realized. In other words, the current applied to the LED module can be kept constant with no restriction on the characteristics or the number of LEDs or a connection arrangement of multiple LEDs.
However, in Reference 3, since an electrically programmable non-volatile semiconductor memory such as a flash memory is needed, manufacturing cost of the LED module increases. Furthermore, it is necessary to provide a signal line for reading the information from and a power line for supplying operational power to the storage unit in Reference 3. This makes wiring for connection between the LED module and the lighting apparatus complicated.